The reactivity of the iron atom in heme proteins is governed both by its coordination geometry and by the degree of rigidity of the protein structure in the immediate vicinity of the heme group. The exact degree to which each of these factors influence the observed rate and type of reaction is only beginning to be understood. We are approaching this problem by examining the reactions of a variety of heme proteins and mode compounds with a homologous series of alkylisocyanides. Since the nature of the iron-ligand and bond is identical any differences between the biding properties observed for these ligands must be due to interactions between the aliphatic side chains and protein residues at or near the distal side of the heme groups. These studies are being supplemented by low temperature laser photolysis experiments which allow visualization of the individual steps in the ligand binding process and by high resolution nmr measurements which yeild structural information about positions of the amino acid and ligand side chains relative to the center of the porphyrin ring. In addition we have been carring out a number of experimental and theoretical studies which are designed to determine the relative importance of unstirred plasma layers, membrane diffusion resistance, hemoglobin concentration and modification, cell size and shape, and anion transport on the gas transport properties of intact erythrocytes. Lastly, heme transport and its incorporation into globin is being examined in terms of the solubility of free heme in membranes and in terms of the role of cytoplasmic transport proteins.